librados: Rollback interface additions

Add C interface rados_ioctx_snap_rollback() and indicate that rados_rollback()
is deprecated.

Add C++ interface IoCtx::snap_rollback() and indicate that IoCtx::rollback()
is deprecated.

Modify snapshot test case to use new function names.

Signed-off-by: David Zafman <david.zafman@inktank.com>
librados: Add ObjectWriteOperation::snap_rollback() for pool snapshots

snap_rollback() is the same as selfmanaged_snap_rollback() but we want an
independent interface for pool snapshots.  Should really take snapname
for consistency with other pool snapshot interfaces.

Signed-off-by: David Zafman <david.zafman@inktank.com>
test: Fix ceph_test_rados to not core dump with invalid arguments

Signed-off-by: David Zafman <david.zafman@inktank.com>
test: Add --pool-snaps option to ceph_test_rados

Fixes: #7940

Signed-off-by: David Zafman <david.zafman@inktank.com>